The editors of "Mechanical Engineering" (Feb. 1992, p. 20) report that "Unsorted plastic waste poses a difficult problem for recyclers, because the polymer materials recovered from these heterogeneous batches of plastics are too low-quality to reuse in high-performance products. For real utility, the various polymer compositions must be separated into pure form." The article then describes two methods of degrading shredded plastics and causing chemical separation into monomers by gas or solvent. These are the alternatives to preliminary extensive physical separation of polymer resin products presented by two prestigious groups, the Rensselaer Polytechnic Institute and the Battelle Memorial Institute.
The Modern Plastics Encyclopedia (R. Martino, ed., Vol. 69, No. 13, December 1992, McGraw-Hill, hereafter referred to as MPE '92) presented an industry overview of plastic recycling techniques and the scope of the recycling problem in the U.S., Japan, and Europe (pp. 33-63, several authors). This exhaustive overview, divided into two sections, "Plastics Solid-Waste Advisory" and "Recycling Technology Update", is part of a product guide for plastics and additives made by the world's major manufacturers.
Information in this type of publication has been closely examined and well-researched by research and industry experts. On page 33, R. Lowman states that, "Plastics were immediately branded as a waste-management villain, largely on the basis of non-degradability even though modern sanitary landfills are designed to retard biodegradation. According to the U.S. Environmental Protection Agency's (EPA) report on characteristics of the municipal solid-waste (MSW) stream, in 1988 plastics accounted for 19.9% vs. 34.1% for paper, universally recognized as a biodegradable material." The table entitled "Plastics sales recycled by selected resins" on page 33 reports that in 1991 only 5% of about 12.5 billion pounds of PET, HDPE, PVC, LDPE, LLDPE, PP and PS was recycled. The rest ended up in dumps and landfills, along with the hundreds of other polymers produced to meet the needs of industry and consumers. The evolution of new plastics to more efficiently meet those needs means that few generalizations can be made about any class of polymers for any length of time. Recycling methods must incorporate the ability to process the diverse formulations of the past, the present, and the future. Listings of engineering and consumer polymers and their additives are typically presented in several volumes of thick books with small type.
In MPE '92, roughly half the pages in the Recycling Technology Update describe systems of physical separation of polymer products to isolate specific polymer resins. Relatively high purity polymer resins must be produced by the separation processes before recycling can be contemplated. Note that on page 49, 8 of the 11 products with post-consumer resins contain no more than 25% of such resins, even though high purity separation has been performed.
On page 54 of MPE '92 begins a section entitled "Chemical Recycling". Chemical or tertiary recycling is a recent innovation, i.e. ". . . was a development welcomed by both suppliers and consumers in 1992." (p. 54). But it then reports that ". . . Existing reclaim and re-use methods are not capable of handling complex waste streams that include wire and cable, medical disposables, blended and compounded plastics, and auto-shredder residues. Other factors influencing the move to alternative recycling procedures have been the quality and performance of recycled plastics." (p. 54) Consistent with the article from Mechanical Engineering above, this section of MPE '92 describes several routes to get crude or pure monomer gases and liquids from reclaimed plastics. The methods include pyrolysis or hydrolysis brought about through high temperature or chemical treatment.
In "Polymer Chemistry" (M. Stevens, Oxford Press, 1990) on page 101 is a Table 3.6 entitled "Types of polyblends" that lists eight technologies to prepare polyblends. Polyblends are defined on page 100 as ". . . any physical mixture of two or more different polymers or copolymers that are not linked by covalent bonds. " In addition to the technologies of Table 3.6, topological bonding, interfacial compatiblizers, and natural affinity interphase bonding are described on page 104 as ways to prepare polyblends. For a compatiblizer polyblend such as ABS (acrylonitrile-butadiene-styrene), the styrene-butadiene and the styrene-acrylonitrile copolymers develop into different crystalline regions held together by grafts. "The number of grafts is small, but is sufficient to provide the necessary interfacial adhesion. ABS polymers are widely used as engineering plastics." (p. 104) Thus, such an important part of plastics technology is maintained on the strength of only a few grafts "to provide the necessary interfacial adhesion".
Polymer Chemistry describes methods for chemical and physical crosslinking on pages 98-100. "Elastomers are characterized by having a very low crosslink density--about one crosslink per 100 monomer units--together with highly flexible main chains to allow large deformations. Elastomeric behavior depends, of course, on polymer structure and crosslink density; but it also depends on morphology and molecular weight." (p. 99). But, in contrast, ". . . Very high crosslink densities lead to embrittlement." (p. 99) Physical crosslinking involves reversible methods of forming hard, solid objects, as opposed to the results obtained with covalently crosslinked resins. "Once crosslinked, a polymer cannot be dissolved or molded." (P. 99)
In "Crosslinking and Scission in Polymers" (O. Guven, ed., Kluwer Academic Publishers, 1990) on page 1, in the article entitled "Molecular Weight Changes and Network Formation by Scission and Crosslinking" (pp. 1-13), A. Charlesby states, "Main chain scission of long chain polymers (degradation . . . ) is to be distinguished from depolymerization . . . (and) involve a random distribution along molecular length, so that whatever the initial molecular weight distribution, it rapidly tends towards a random molecular weight distribution . . ." And on page 2, he adds that, "Fracture of the main chain leaves two radicals at the side of the scission. Which chemical changes occur to stabilize these highly reactive ends?"
In the Charlesby article, crosslinking is discussed on pages 4-5--"The effect of such bonds (crosslinks) depends not only on their density and on the chemical structure of the individual bonds, or even on the molecular mobility and morphology, but also on the distribution of the crosslinks . . . Many of these physical properties dependent on crosslink density are also influenced by chain entanglements, which can behave for limited periods as equivalent to permanent crosslinks. The mechanical behavior of a crosslinked and entangled network will be a composite of permanent elongation plus flow, the latter being due largely to the changes in entanglements under stress."
U.S. Pat. No. 4,013,616 (Wallace '616) describes using ". . . an unpyrolyzed, unincinerated, comminuted fraction from a front end recovery system including glass, cellulose and its derivatives, inorganic oxides, and mixed polymer resins." (col. 2, ll. 6-9) as a filler for ". . . flowable, castable thermoplastic or elastomeric polymer." (col. 2, ll. 5-6). A glass-rich fraction must be used to overcome inherent incompatibility of mixed polymers. Wallace '616 states that, "One reason why the shredded polymer-rich residue from a front-end system would form a poor material by itself is that it contains a number of mixed polymers which are incompatible with each other, for example, polyethylene and polyvinyl chloride. When these polymers are melt blended, the product appears striated or layered, is fibrillated and has inferior physical properties and poor structural integrity. When blends of these polymers are mixed without segregation, they have poor elongation properties and are cheesy and brittle." (col. 1, ll. 44-54). The solution offered by Wallace '616 is to dilute incompatible filler and binding resin with ground glass so that ". . . it can form an interface with the binder of sufficient length to provide a significant weakened fault line in the composition." (col. 2, ll. 35-36) In other words, the solution is dilution instead of establishing compatibility. The apparent amounts of mixed polymeric resin used in the final product appears to be no higher than about 2-8 weight percent with virgin resin binder at some higher percentage. There appears to be no general indication that higher levels of mixed polymeric resin would be achievable by prior art methods.
German Patent Application 4102235--A requires that a floatable portion of domestic waste be further separated to remove all polymers but thermoplastic polyolefins, such that a thermoplastic elastomer is made therefrom. There is no teaching that any fiber or mineral filler be used in the final product. There is no teaching that extruder processing is used. A natural rubber must be used with dynamic vulcanization to produce a usable product. Both German Patent Application 4102235--A and Wallace '616 require specific separations of the waste portions to be used. A floatable portion of waste as described in those references is not amenable to use without specific direction concerning its separation and subsequent processing. One reference process produces a glass and fiber filled product with low mixed resin content, while the other reference process produces a thermoplastic elastomer with no contamination from other polymers permitted. This combination of information would hardly lead the skilled person to select a general floatable portion, of which Wallace '616 identifies at least 3 different portions obtainable from floatation, as a standard commodity in combining with other polymers.
An abstract of a Japanese patent publication, JP 5 6106-939 (25.08.81), to Nisshin Kogyo KK (Kogyo '939) discloses a "low fluidity" and "crosslinked" polyethylene. "Low fluidity" polyethylene must be distinguished from "no fluidity" polyethylene. Thermoset material, by definition, has no fluidity, is not flowable and is generally known by its ability to recover its original shape after heating, compression or deformation. "Low fluidity" at "low temperature", as used in Kogyo '939, describes polyethylene not completely thermoset--either the melt index of the material is not zero or it contains substantial amounts of polyethylene with a melt index above zero. The essential nature of a thermoset material is that of a single, large macromolecule which will not become fluidized unless substantial breaking of its covalent bonds is effected. The disclosure of Kogyo '939 is insufficient to discern whether a useful product is made or how to proceed to obtain the indicated product. The abstract does not inform the skilled person of the "low" temperature range, relative amounts of polyethylene (I) and polyolefin (II) and the length of the "short" retention time. Kogyo '939 is an invitation to expend considerable time and experimentation in determining how to actually use the material in injection molding, blow molding or rotomolding. Polyolefin (II)'s are used in the alternate "or" for individual mixing with the "low fluidity", "crosslinked", waste polyethylene, so that only a mixture of "low fluidity" polyethylene with polyethylene or polypropylene is mentioned. The broadest blend of polymers described in Kogyo '939 is that of polyethylene and polypropylene, a well know composite in the prior art.
The editors of the Economist (Oct. 1, 1994, p. 109) in an article entitled "The go-betweens" state that, "Plastics are prime candidates for recycling, but that often means making do with a mixture of different polymers, some of which do not fit well together well. These re-melted mixes have a tendency to harden in brittle, weak solids with unpredictable properties." They then describe a recent polyblending innovation using polystyrene-enhanced PEA and PMMA in a new alloy-type polymer. The attraction of the PS portions permit separate crystalline regions of PEA and PMMA to interfacially mesh and bond to create a new polymer with properties that would have been unpredictable from the properties of either PEA, PMMA or their PS-enhanced derivatives. There is no indication that the enhancing moieties, the potential polyblending candidates, or the properties of the resulting polyblend could be further predicted from the innovation described in this reference.
As noted above, the potential for reclaiming plastics from landfills means that the outdated formulations of the past, the bewildering array of additive-enhanced formulations of the present, and the certainty that plastics will continue to become more complex in the future means that a process with broader application that those of the prior art must be found to improve the profitability of such reclaiming.
An abstract of a German patent publication, DE 3239-526-A (26.04.84) to RXS Schrumpftechnik (RXS '526) describes a "crosslinked" polymer as being 90 percent or less of "granules" used in a mixture in which those "granules" will comprise 50 percent or less of the mixture. The "granules" will not have thermoset properties whether or not the "crosslinked" polymer is actually thermoset. Addition of 10-90 percent of non-crosslinked polymer will give the mixture a flowable quality uncharacteristic of thermosets and necessarily infers some non-standard process step to obtain those "granules". The origin or processing of the 10-90 percent non-crosslinked polymer in relation to the "crosslinked" polymer is not disclosed in RXS '526. It would cause the skilled person to refrain from using the process of RXS '526 as a teaching until the nature of that non-crosslinked polymer and the processing steps to get it were disclosed. In addition, RXS '526 merely indicates that a "non crosslinked polymer" is suitable for mixture with the "crosslinked polymer". Although those skilled in the art of polymers have some ability to discern direction from the way language is used in a reference, the failure of RXS '526 to tell what a "non crosslinked polymer" is at the point of asserted novelty renders the reference of little use as a teaching. In RXS '526, the "crosslinked" polymer will thus always be less than or equal to 45 percent of the mixture.
In addition, in RXS '526, "crosslinked" polymer must be mixed with "non crosslinked polymer" granules, injection molded and then irradiated to cause sufficient crosslinking so that the properties of the mixture are the same as those of the "non crosslinked polymer" material. A usable product is obtained only as a fully crosslinked (by irradiation) polymer. As demonstrated by the above prior art, incompatibility of polymer resins mixed by melting generally results in a useless material. The wide range of formulations of additive-enhanced products from even a single type of polymer resin mean that such information as presented in RXS '526 is not useful other than to invite a fairly long and expensive course of experimentation to establish compatibility of crosslinked polyolefin waste with whatever might be meant by the term "non crosslinked polymer". There is no description in RXS '526 concerning crosslinked polyolefin waste. Virtually every polyolefin resin is crosslinked to some degree. Crystalline, semi-crystalline and amorphous structures are found in thermoplastics, elastomerics and thermosets. Without indication concerning the degree of crosslinking and at least the category of crosslinked polymer resin, RXS '526 leads the skilled person to contemplate undue experimentation to establish its usefulness.
U.S. Pat. No. 4,255,372 (Kuhnel et al '372) describes making a ". . . `homogenous foamed article`. . . understood to mean a foamed article which again exhibits closed cells of uniform structure." (col. 1, ll. 45-47). A crosslinking agent is used on foam scraps such that the ". . . bulk density of the shaped article produced from the foam scraps, though, is increased over that of the original foam material. One explanation for the invention could be seen in that there are residues of blowing agent and residues of crosslinking agent in the foam scraps stemming from the original manufacturing process, which agents have not as yet been decomposed and are activated during the renewed expanding and crosslinking process." (col. 1, ll. 52-60) "Crosslinked polyolefin foam chips" (col. 5, ll. 48) are described as being processed with pure high pressure polyethylene with a melt index of 4 g./10 min. or pure ethylene-vinyl acetate copolymers at less than 25 weight percent foam chips. A first mixing in a hot mixer, rolling mill or extruder must take place at less than the decomposition temperature of a crosslinking agent. Only polyolefin foam scraps containing elastomer or synthetic resin and having been made with a peroxide crosslinking agent and a blowing agent can by processed by the method of this patent.
U.S. Pat. No. 3,806,562 (Lamort et al '562) appropriately states that a ". . . difficulty in the reuse of scrap plastics arises from the fact that these latter, like any other material being recovered, originated from batches of variable composition from which is necessary to obtain a product of substantially constant characteristics." (col. 1, ll. 62-66) The process of Lamort et al '562 uses a mixture of low softening temperature thermoplastic polymer resin and other "non-softened particles" (col. 2, ll. 9-10) in a melt blended paste, into which mineral fillers could also be added. There are no specific examples of the low softening point polymer resins to be used. There is no mention or indication that compatibilizing, as defined in the selection from Polymer Chemistry, has occurred, nor is there any teaching concerning the melt blending of dissimilar polymer resins, as warned of in Wallace '616. Mere dilution of low softening point thermoplastic polymer resin with non-reactive, non-polymer particles may be effective up to the point that mechanical integrity of the crystalline structure is affected. But that is not true of particles of polymer resins--all such particles have strong or weak affinities and/or repulsions for other polymer resins. Even though greatly diluted, certain end groups and a small number of grafts and branches of a polymer molecule dramatically influence its compatibility with other melt blended polymer resins.
U.S. Pat. No. 4,098,649 (Redker '649) describes a method for volatilizing organic matter, i.e. paper, wood, plastic and vegetable matter, in the absence of oxygen. This is a continuous destructive distillation method analogous to pyrolytic monomer recovery and is not directed to any melt blending or compatibilization of polymer resins.
Two publications disclose using homogenous, 0.962 density HDPE with a narrow melt index (5-8) ground to from 35 to 100 mesh in a melt blend with virgin HDPE for compression molding and injection molding. The two publications are (1) a paper titled "Recycling Cross-Linked Polyethylene" by Elmer Good, presented to the Association of Rotational Molders on Oct. 7, 1991 and (2) a report of that presentation in Technology News for June 1992, pp. 43-44, by J. Ogando. The Good paper discloses that the above previously processed (blow molded) HDPE was "crosslinked". Prior art methods of crosslinking HDPE include rotomolding or reaction molding with a crosslinking agent, although there is no disclosure concerning either the process or the degree of crosslinking achieved for "crosslinked" material. It is further disclosed that the homogenous, "crosslinked" material was used for injection molding and rotomolding when combined with 65-75 weight percent virgin HDPE with closely matching densities and melt indices of the blowmolded HDPE before it was "crosslinked". It was an essential requirement of the process in the Good paper that the "crosslinked" material be pulverized to between 35 and 100 mesh at considerable cost before mixing with the virgin HDPE.
The Ogando paper shows a microphotograph (improperly labeled as an injection molded product) of the blend of the "crosslinked" HDPE with the virgin, density-matched HDPE in a rotomolded product. Proper rotomolding processing will produce a HDPE product whose melt index is zero, i.e., a thermoset. It appears that the photograph in the Ogando article shows a product using the Gyron compound, which in the Good paper is described as being used only in rotomolded samples.
For the Good and Ogando papers, it is critical to note that in none of the samples where HDPE is "recycled" was "scrap" from rotomolding or reaction molding used. The assertion of the Good paper that "scrap" from the operations that typically produce unusable, thermoset scrap, i.e., rotomolding and reaction molding, could be injection or rotomolded was not demonstrated. It was asserted in the Good paper that 0.962 HDPE solely from blow molded products with a low and narrow melt index range (typical of blow molding polymers) with an undisclosed degree of crosslinking would blend with virgin HDPE with a matching density and melt index range for use in injection molding or rotomolding.
The results reported by the Good paper appear to be consistent with the requirements (1) in RXS '526 that "granules" that will later be used with "non crosslinked polymer granules" have less than 90 percent of some undescribed "crosslinked" polyolefin and (2) in Kogyo '939 that (in two places in the short abstract) "low fluidity" polyethylene which has been to some degree "crosslinked" can be extruded with LDPE, HDPE or polypropylene at a low temperature. The sum of the teachings of the prior art contain no suggestion that a material which is thermoset can be blended with a thermoplastic without first melt blending a non-thermoset polymer with it unless the mixture is diluted with mineral fillers or fibers.
In addition, the consistent teaching of the prior art in U.S. Pat. No. 5,215,695, German Patent Application 4105285-A, and U.S. Pat. No. 4,013,616 is that when thermosets are mixed with thermoplastics, a mineral filler or fiber must be added with it. The person making skilled choices in selection of processes for using thermosets with thermoplastics is nowhere in the prior art clearly given a suggestion to use thermosets with thermoplastics unless something else is added with them. Instead, in each case in the prior art, the "crosslinked" material was in some sense diluted with thermoplastic or was not clearly thermoset prior to mixing with thermoplastics, or the "crosslinked" material was diluted with an inert filler such as mineral filler or fibers during the melt blending process.
U.S. Pat. No. 5,215,695 describes shredding a cellulose-backed laminate whose aminoplastic resins are not fully condensed to form a laminate scrap powder mixture. The aminoplastic resins are not thermoset before they are powdered. The aminoplastic resins do not become part of a thermoset product. The aminoplastic resins become further condensed upon mixing with thermoplastic, although it is not possible to discern whether the aminoplastic resin portion becomes thermoset in the final product. The cellulose portion of the scrap powder is about 63 weight percent. Phenolic and melamine resins make up the balance. A set of thermoplastic polymers described in col. 4, ll.9-14 are added to an extruder where additional condensation takes place. An extruder is described as a preferable place for mixing and melting the resins for this process of this patent. There is no indication that mixing and melting resins other than those described in the patent for the process described therein would be effective or useful for those other processes or resins. Only a single, short sentence in the patent refers to the operation of the extruder specifically. The requirement of a cellulose content of at least about 30% in the final blend indicates that the teaching of using an extruder for mixing and melting in the art is very limited.
German Patent Application 4102235-A describes using a mixer for thermoplastics, thermosets and "particularly" mineral and natural fibers prior to feeding to a heated extruder. Consistent with the teaching of U.S. Pat. No. 5,215,695, the extruder is used only in conjunction with high fiber content in relation to the polymer component. It nowhere indicates other uses of the extruder. Used car parts are proposed to be "disintegrated" and then the particles are separated by origin into separate bins, i.e., "according the car component being utilized". The use of each car component is not on an unsorted basis and is effectively controlled by variable mixing of portions from four different bins. German Patent Application 4102235-A and U.S. Pat. No. 5,215,695 teach careful control over materials being fed to a heated extruder. There is no generalized invitation to use heated extruders to perform any sort of polymer blending, whatever the type of polymer or product to be made.